1. Field of the Invention
The present invention relates to an optical semiconductor device, and more specifically, to an optical semiconductor device for forming a neural network.
2. Description of the Related Art
An information processing system using a neural network is one of sophisticated parallel distributed information processing systems having learning capability which simulates the information processing in the brain. Such an information processing system having a neural network is excellent in high-speed pattern recognition and knowledge processing based on incomplete data. On the contrary, Neumann type serial information processing systems which are in major use at present are inferior for conducting such types of information processing. The information processing system having a neural network is therefore expected to be a system which can overcome the above and other disadvantages of the Neumann type serial information processing systems, and is one which has been intensely studied.
FIG. 8 schematically shows the operation of a neuron 100 to be used in a neural network. The operation of the neuron 100 is represented by formula: ##EQU1##
The neuron 100 receives an input signal Si (i=1 to N) and outputs an output signal x. The input signal Si is synaptically interconnected with the neuron 100 with a strength of wi (i=1 to N) which is a weight indicating the strength of the synaptic interconnection. When the weight wi is a positive value, the synaptic interconnection is excitatory. When the weight wi is a negative value, the synaptic interconnection is inhibitory. When the weight wi is zero, there is no synaptic interconnection. When the sum .SIGMA. (Siwi) of the products of the input signal Si and the weight wi exceeds a threshold level h, the neuron 100 is made excited and outputs the output signal x. Varying the weight wi is called learning. When the weight wi varies in response to the input signal Si, the learning is called self-learning.
In order to form a neural network to complete an information processing system, a number of neurons identical to the neuron 100 are required, and they must be mutually connected. More concretely, an output signal x from another neuron not shown must be supplied to the neuron 100 as the input signal Si. As the number of neurons constituting the neural network increases, higher-level information processing is possible. However, if a number of neurons are to be mutually connected through conventional electrical wirings, the number of electrical wirings required is so enormous that it is difficult to complete the neural network using such electrical wirings. This is especially true when the neural network is composed of neurals arranged with high density.
In order to solve the above problem, the use of light for the interconnection among a plurality of neurons has been studied. For example, IEEE Photonics Technology Letters, vol. 4 (1992), pp. 247-249 describes an optical neurochip made of semiconductor material having a light emitting device and a photodetector. FIG. 9 schematically shows such an optical neurochip 200 including a light emitting diode (LED) array 201 and a photodetector array 202. The LED array 201 consists of LEDs 203 arranged in a matrix with eight lines and eight rows. Each of the LEDs 203 includes a multi quantum-well active layer 204 and a distributed bragg reflector 205. The photodetector array 202 consists of photodetectors 206 each of which is arranged at a position corresponding to each of the LEDs 203. The photodetectors 206 having an MSM (metal-semiconductor-metal) structure are fabricated by evaporating aluminium on a GaAs substrate 207 to form electrodes 208. The electrodes 208 are connected to wire bonding pads 209 formed on the GaAs substrate 207. Bumps 210 are formed on the GaAs substrate.
In the optical neurochip 200, the eight LEDs 203 in each line simultaneously emit light having an identical intensity as the input signal Si shown in FIG. 8. This corresponds to supplying an output from one neuron to other neurons simultaneously as signals having an identical intensity. The light from the LEDs 203 is received by the corresponding photodetectors 206. In each of the photodetectors 206, a voltage is applied to one of the electrodes 208 thereof from an external source through the wire bonding pad 209. The sensitivity of the photodetectors 206 is adjustable by varying the level and the polarity of the applied voltage. This corresponds to adjusting the weight wi of the synaptic interconnection shown in FIG. 8. Each eight of the photodetectors 206 are mutually connected in a row so that the sum of photocurrents flowing in the eight photodetectors 206 can be taken out. This corresponds to obtaining the sum .SIGMA.(Siwi) of the products of the input signal Si supplied from neurons and the weight wi of the synaptic interconnection. Thus, the optical neurochip 200 has realized the synaptic interconnection.
However, the optical neurochip 200 has disadvantages as follows: First, threshold processing is not available. It is required, therefore, to provide an external operation circuit to conduct the threshold processing of the signals output from the optical neurochip 200 before being supplied to another optical neurochip. Second, since the input signals and the output signal of the optical neurochip 200 are electrical signals, electrical wirings are required for the formation of a neutral network. Thus arises the same trouble as described above.
An optical semiconductor device having a photodetector and a light emitting device is described in the Institute of Electronics, information and communication Engineers Technical Report, OQE-91-53 (1991), pp. 45-50. Referring to FIG. 10, an optical semiconductor device 250 includes a semiconductor laser 251, an absorbing layer 252 formed on the semiconductor laser 251, and heterojunction phototransistors 253, 254, and 255 formed on the absorbing layer 252. The semiconductor laser 251 includes an undoped active layer 256 having a band gap of 1.3 .mu.m. Each of the phototransistors 253 to 255 includes a collector layer 257, a base layer 258, and an emitter layer 259, which are doped with impurities at concentrations of 1.times.10.sup.17 cm.sup.-3, 5.times.10.sup.16 cm.sup.-3, and 1.times.10.sup.18 cm.sup.-3 respectively. The band gap of the base layer 258 is 1.2 .mu.m. The phototransistors 253 and 255 are formed at positions shifted from the center of emission of the semiconductor laser 251 by a horizontal distance of 185 .mu.m opposite to each other. The phototransistor 254 is formed above the center of emission of the semiconductor laser 251. The absorbing layer 252 includes a first absorbing layer 260 having a thickness of 1 .mu.m and a band gap of 1.2 .mu.m and a second absorbing layer 261 having a thickness of 1 .mu.m and a band gap of 1.3 .mu.m.
In the optical semiconductor device 250 having the above-described structure, when the phototransistor 253 is irradiated with an input beam 262, a photocurrent is generated. The photocurrent then flows into the semiconductor laser 251 and causes laser oscillation. In general, a semiconductor laser emits weak light even when the current flowing therein is too small to cause laser oscillation. In the optical semiconductor device 250, therefore, if the semiconductor laser 251 emits weak light without generating laser oscillation, the emitted light will be absorbed into the phototransistors 253 to 255 as feedback light, resulting in production of a large photocurrent. This large photocurrent will cause the semiconductor laser 251 to emit light more intensely. With this positive optical feedback, the semiconductor laser 251 will finally oscillate, which will prevent the semiconductor device 250 from conducting normal optical amplification operations.
In order to solve the above problem, the active layer 256 of the semiconductor laser 251 is formed of a semiconductor material different from that for the base layers 258 of the phototransistors 253 to 255. Thus, the wavelength of light oscillated by the semiconductor laser 251 is different from the detection peak wavelength of the phototransistors 253 to 255. Further, the absorbing layer 252 is provided in order to minimize the influence of internal feedback light.
With the above structure, however, the phototransistor 254 formed above the center of emission of the semiconductor laser 251 still receives internal feedback light. Therefore, in order to substantially eliminate the influence of internal feedback light, the phototransistor 253 (or 255) which is formed at a position shifted from the center of emission of the semiconductor laser 251 is used as the photodetector.
Thus, in the optical semiconductor device 250, the phototransistor 253 is used as the photodetector, and the current is biased by dark current to a level slightly lower than that at which the semiconductor laser 251 starts oscillating. Under these conditions, when an input beam 262 is incident to the phototransistor 253, the semiconductor laser 251 oscillates and outputs an output beam 263. Since the semiconductor laser 251 has an output light-current characteristic with good linearity, an output beam 263 with high intensity can be obtained even when the input beam 262 is weak, allowing the optical semiconductor device 250 to conduct the optical amplification operation.
However, the optical semiconductor device 250 is still disadvantageous for use as a neuron constituting a neutral network for the following reasons:
The wavelength of the light emitted from the semiconductor laser 251is different from the detection peak wavelength of the phototransistor 253. Accordingly, when the output beam 263 is introduced to a phototransistor of another optical semiconductor device as an input beam, the detection sensitivity lowers, and therefore it is difficult to form a neural network with effective signal transmission. Further, since the phototransistor 254 formed above the center of emission of the semiconductor laser 251 cannot be used as the photodetector, high-density integration of the optical semiconductor device 250 cannot be realized.